Due to the rise of an inverter technique whose power supply use efficiency is improved, an application of the inverter technique spreads into general electric appliance. An inverter instrument realizes improved efficiency and reduction in size and weight of a power supply transformer by shifting the switching frequency to a high frequency. However, with respect to an electromagnetic noise, since a number of high level harmonics are generated accompanied with switching, it is general that power supply filters are installed for preventing electromagnetic interference to the other appliances. However, these power supply filters are designed considering only the condition that the instrument is continuously operating. It is usual that power is applied to instruments when only it is necessary to operate while the power supply is cut off to stop the operation when it is unnecessary to operate, with considering efficiency of power supply utilization or energy saving, and accordingly turning on/off of the power supply occurs more frequently than in the past.
As a technique for reducing a rush current generated when power is turned on, for instance, the Japanese Laid-Open Patent Publication No. H05-19879 discloses a technique for making current (drain current) flowing through a power supply line constant, by adjusting its gate voltage with a voltage divider while inserting a MOSFET in the power supply line.